Bulk carbon and lignin fingerprinting of catchment sediments transported by mountain rivers in Nepal Himalayas

Abstract

The Himalayan rivers yield the most significant flux of continental sediments into the ocean. Organic matter (OM) transported by these rivers provides a peek at the influence of diverse geological terrains, soil types, vegetation, and climate on carbon cycling within a narrow boundary. We analyzed suspended and bedload sediments from four Himalayan rivers to trace their sources, elucidate their fate during fluvial transport, and estimate the organic carbon (OC) flux. Hence, total OC (TOC), dissolved organic carbon (DOC), C:N ratios, and lignin phenols were measured. Consistent with the erosional intensity in the rivers, suspended sediment load input followed the order: Kaligandaki > Myagdikhola > Aadhikhola > Tinahukhola. C:N values in rivers from the Lesser Himalayas and Siwalik indicate sediments from mixed biogenic sources. In contrast, high TOC and C/N values in the trans-Himalaya rivers flowing through barren landscapes reflect the erosion of catchment sediments yielding petrogenic carbon. The suspended matter in rivers from the Lesser Himalayas and Siwalik has higher lignin phenol concentrations than the trans-Himalaya and Higher Himalaya rivers. The lignin phenol ratios indicate higher degradation in rivers from the trans and Higher Himalaya sections. This implies that only a small fraction of the terrestrial OM transported by these rivers deposits in the ocean sink. In contrast, rivers from the Lesser Himalayas and Siwaliks sequester a significant amount of OM bound to their bedload. As a result, these rivers transferred lower particulate OC (POC) but higher DOC than similar rivers worldwide. Rivers from Lesser Himalayas and Siwaliks transfer > 90 % of annual POC flux during monsoons. Finally, although Himalayan rivers transport less OC than other global rivers traversing densely vegetated landscapes, the sheer number of these rivers has significant implications on the fate and transport of total OC from catchments sediments.